[media] media: radio: correct module license (==> GPL v2)

As per code header comment, changing the driver license entry
to match the correct version.

Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>